Multi-mode 850-nm VCSELs based upon the AlGaAs materials system have been the standard optical source for application to glass optical-fiber-based data communication links. However, the implementation of VCSELs for other applications frequently requires different wavelengths and performance attributes than those for which the 850-nm multi-mode VCSELs have been optimized.
As a first example, a very important optically based non-invasive medical-sensor application is oximetry. Pulse oximetry is well-established, and tissue or regional oximetry is an emerging application. Both of these versions of oximetry take advantage of the varying absorption coefficient as a function of wavelength for different types of hemoglobins, (i.e., oxyhemoglobin, reduced hemoglobin, carboxyhemoglobin, or methemoglobin). The sensors rely on the absorption of wavelengths in the regime from about 660 nm to 1000 nm, and as the number of hemoglobin types that are to be analyzed increases, the number of different wavelengths that are required also increases. These applications benefit from the narrow spectral linewidth and the slow spectral shift with temperature of the VCSEL, while wireless implementations benefit from the reduced power consumption of VCSELs as compared to light emitting diodes (LEDs). However, to be commercially viable, VCSEL wavelengths spanning the range from red to near-infrared are required, and in particular, red wavelengths are an important aspect of these systems.
A second example application is plastic optical-fiber links based on poly(methyl methacrylate) (PMMA) fiber materials that have been implemented for sensor and data links in automobiles, and are currently being considered for use in home networks. In some embodiments, PMMA-based fiber has a primary-absorption minimum in the green-wavelength region, and a secondary absorption minimum in the red-wavelength region. Absorption at 850 nm is too high to be practical for typical plastic fiber links having lengths of more than just a few meters.
There is a need for improved laser systems, and in particular, there is a need for VCSELs having improved device characteristics, improved manufacturability, expanded wavelength coverage, improved performance, improved reliability, and improved performance, and additional packaging techniques in order to provide commercially viable devices, products, and systems.